This invention relates generally to the fabrication of semiconductor microelectromechanical structures (MEMS) and more particularly the invention relates to the fabrication of structures such as mechanical resonators which require narrow gaps between a resonator body and drive and sense electrodes.
Micromachined resonators are promising as on-chip replacements for discrete filters and oscillators which are presently implemented using off-chip quartz and surface-acoustic-wave (SAW) devices in wireless communication systems. The monolithic integration of control, amplification and signal processing electronics with radio frequency microelectromechanical devices (RF MEMS) also reduces parasitic capacitances and inductances as well as overall system power consumption. Recent progress toward a low-thermal-budget polycrystalline silicon-germanium (poly-SiGe) MEMS technology promises to make possible the modular co-fabrication of high performance surface-micromachined structures on top of completed CMOS wafers. However, a detailed process integration scheme for poly-SiGe resonator fabrication—particularly one that provides ultra-narrow lateral gaps between the resonator and drive/sense electrodes—is needed.
There are several process requirements for post-CMOS integration of RF resonators. First, the MEMS fabrication thermal process budget must be carefully designed in order to retain the performance and reliability of the CMOS electronics. Interconnect reliability, rather than transistor performance, limits the post processing temperature, which should not exceed 450° C. High quality poly-SiGe structural layers with Ge content greater than ˜60% can be deposited by conventional low-pressure chemical vapor deposition without exceeding this temperature limit. Second, the metallization stack of the underlying electronics must be protected from chemical attack during the microstructure release etch. The use of Ge rather than SiO2 as the sacrificial material is advantageous because it can be rapidly etched in a heated H2O2 solution which does not attack oxide, metal, poly-Si, or poly-SiGe with a Ge content less than ˜70%. Third, extremely small (sub-100 nm) electrode gaps are needed in order to achieve low motional resistances, hence low insertion losses. For the definition of nanoscale lateral gaps, a sidewall sacrificial oxide spacer process has already been developed for poly-Si MEMS technology. In this process, the poly-Si electrode layer (which is deposited after the sacrificial spacers are formed) must be etched with high selectivity to the sacrificial oxide, in order to clear it completely along the sidewalls of the structural poly-Si layer in the areas where the electrode layer must be removed. Because Ge etches more quickly (by a factor of ˜3) than poly-SiGe or poly-Si in a standard Cl2/HBr dry-etch chemistry, the sidewall spacer process cannot be used if Ge is employed as the sacrificial material, however.